Chemical polishing of aluminum and aluminum alloys



United States Patent 3,425,881 CHEMICAL POLISHING 0F ALUMINUM AND ALUMINUM ALLOYS Charles C. Cohn, Atlantic City, N.J., assignor to Samuel L. Cohn and Charles C. Cohn, co-partners trading and doingPbusiness as Colonial Alloys Company, Philadelphia, a.

No Drawing. Filed June 28, 1965, Ser. No. 467,698 US. Cl. 156-19 8 Claims Int. Cl. C23g 1/12 This invention relates to the chemical polishing of aluminum and aluminum alloys, which are herein referred to collectively as aluminum. More particularly, it relates to chemically brightening articles of aluminum in a bath and also to the replenishing of that bath and maintaining the bath for continued treatment of aluminum articles.

The chemical polishing of aluminum is described in my Patent 2,729,551 dated Jan. 3, 1956 in which there is disclosed the use of polishing baths containing upwards of about 40% phosphoric acid and upwards of about /2% of nitric acid, by weight. In accordance with the patent, optimum specular effects are secured when the percentage of phosphoric acid is relatively high (for example in excess of 65%). For producing specular finishes, such baths are operated at a relatively high temperature ranging from about 180 F. to the boiling point of the bath. Times of immersion of the work may vary from a few seconds to around three minutes or more.

It is an object of this invention to brighten aluminum surfaces so that the image clarity is greater than that achieved by any method of the prior art.

It is another object of this invention to achieve superior brightening of aluminum articles while allowing at least 30 seconds for transferring the article from the brightening bath to a rinse bath or spray.

It is another object of this invention to maintain the brightening bath in such a manner as to extend the life of the bath almost indefinitely.

It is another object to eliminate the over-accumulation of dissolved aluminum in the bath.

It is another object of this invention to provide a lowcost polishing bath which polishes aluminum articles to a high degree of image clarity.

As referred to herein, initial or starter bath is defined as the fresh bath which is made at the start of the brightening operations, and operating bath is the bath after the brightening operation has been in progress. The composition of the operating bath changes during use so that it requires replenishing to bring its volume and proportions of constituents back to approximately the same as the starter bath. Replenish solution is' defined herein as the solution of ingredients added to the operating bath to adjust for all or some of the changes made therein by operation.

To illustrate what is meant by the superiority of image clarity by the present invention, aluminum articles were brightened by the best-known prior art method and were compared with aluminum articles brightened by the present invention.

The aluminum articles were made of aluminum alloys numbers 5252-H 25, 5257, 5457, 5557-0 Temper, and 5657-H 25.

The composition, by weight, of the prior art operating bath was as follows:

Free H PO percent 63.0 to 69.0 HNO do 3.0 H 0 -..do- 19.0 to 22.0 Aluminum do 1.98 to 2.64 Brightening temperature F About 210 3,425,881 Patented Feb. 4, 1969 Brightening time minutes About 1 to 3 Transfer time seconds- Not over about 30 (The ranges represent variations of the bath during prolonged use.)

A similar set of aluminum articles were brightened in accordance with a bath of my invention in which the composition of the initial bath, by weight, was as follows:

H PO percent 42.60. H2804 dO 20.80. HNO do H O do 18.95. Aluminum do Ammonia do 0.244. Copper (as metal) do 0.0118. Specific gravity (at 25 C.)

About 215 F. to about 235 F. (the temperature best suited to get optimum brightening for the particular alloy being treated).

About 2 minutes.

From 30 seconds to several minutes or more.

It is to be noted that the above prior art bath does not use sulphuric acid, and is accordingly a more costly mixture because of the high price of phosphoric acid as compared to sulphuric acid. Some prior art baths do employ sulphuric acid. However, in prior art baths, the transfer time from brightening bath to the rinse bath is limited to about 30 seconds since a transfer time in excess of that will result in the etching of the aluminum articles. As a practical matter, it is very difiicult to transfer an aluminum article from the brightening bath to the rinse bath in less than 30 seconds. My initial or starter bath contains a substantial amount of aluminum in solution in order to get optimum brightening from the very start.

Comparative image clarity measurements were made on a Hunter Distinctness of Image Glossmeter Model D36 in accordance with Ternstedt Test Method #1064, spec. TP-355. The following results were obtained.

Brightening temperature:

Brightening time Transfer time Image Clarity My superiority,

An example of the preparation of an initial bath for optimum brightening of aluminum articles in accordance with this invention is as follows: 6.5 liters of 42 B. nitric acid are added to 64.4 liters of phosphoric acid at a temperature of about P. Then 7.7 kilograms of alumina are added which become dissolved in the phosphoric and nitric acids. Then 28.6 liters of 66 B. sulphuric acid plus 1.8 kilograms of ammonium sulphate and 0.085 kilogram of cupric nitrate are added to the mixture. Then the specific gravity of the bath is adjusted to 1.745 at 25 C. by adding water. Then the bath is heated to about 215 F. and the aluminum articles are immersed in the bath.

The ingredients of this bath are as follows before and after reacting, by weight (the proportions of the ingredients measured before mixing are listed under Before Reacting and the proportions after mixing are listed under After Reacting. The proportions change after mixing because of reaction between ingredients. For example, the alumina or aluminum reacts with phosphoric and sulphuric acids to form salts of these acids and thereby reduce the proportion of these acids in the bath).

Before reacting, After reacting, percent percent H3PO4 51.087 47. 90 H2501... 27.140 24.30 HNOg... 3.481 3.42 Aluminum (as metal) 1. 39 1. 64 H 13. 412 15. 73

0.0122 0.0122 Ammonia 0. 258 0. 258

EXAMPLE 2 Bath compositions of the present invention which are preferred for optimum image clarity may be stated to have approximately the following ranges of constituents after reacting:

H PO percent 42.5 to 48 H2SO4 dO t0 HNO do 3.0 to 3.6 Aluminum (as metal) do 1.6 to 2.7 H 0 do 15.6 to 19.0 Copper (as metal) do 0.0116 to 0.0126 Ratio of H PO to H 80 1.57:1 to 2.07:1 Total H PO and H 50 percent 63.4 to 72.2 Specific gravity (at 25 C.) 1.74 to 1.75

EXAMPLE 3 Bath compositions in accordance with the present invention which brighten a wider range of aluminum alloy compositions than the immediately preceding bath compositions are as follows: H PO percent 40.0 to 64.6 H 80 do 9.97 to 25.0 HNO do 1.5 to 4.0 Aluminum (as metal) do 1.45 to 3.1 H 0 do 13.20 to 21.5 Copper (as metal) do 0.0116 to 0.0126 Ratio of H PO to H 1.57:1 to 6.521 Total H PO and H 80 percent 59.0 to 74.0 Specific gravity (at 25 C.) 1.72 to 1.77

EXAMPLE 4 45 For excellent image clarity superior to that obtainable by the prior art methods, bath compositions having the following ingredients in approximately the indicated ranges may be used for all of the aluminum alloys which may be considered to be subject to brightening; i.e., those alloys 0 which do not contain a high percentage of silicon for example:

H PO percent 40.0 to 64.6 H 80 do 9.97 to 25.0 HNO do 1.5 to 4.0 Aluminum (as metal) do 1.45 to 3.] H O do 13.7 to 21.5 Copper (as metal) d0 0.0116 to 0.0126 Ratio of H3PO4 t0 H2804 to Total H PO plus H 50 percent 50.0 to 89.6 Specific gravity (at 25 C.) 1.72 to 1.77

EXAMPLE 5 Bath compositions in accordance with the invention which may be used to give optimum image clarity for most, but not all, of the aluminum alloys which may be brightened comprise approximately the following ranges of active constituents:

H PO percent 42.5 to 48.0 H SO do 21.0 to 25.0 HNO do 3.0 to 3.6 Aluminum (as metal) do 1.6 to 2.7 H 0 do 15.6 to 9.0

Copper (as metal) do 0.0116 to 0.0126 7 4 Ratio of H PO to H 50 1.57:1 to 2.08:1 Total H PO plus H 50 percent 63.5 to 73.0 Specific gravity (at 25 C.) 1.74 to 1.75

In both Examples 4 and 5, small amounts of ammonium salts, for example, about 1% by weight, in the form of ammonium sulfate may be added to the bath.

The baths of both Examples 4 and 5 are operated at about 210 to about 235 F. and are preferably heated to within the range of 210 to 220 F. The time that the aluminum articles are immersed is on the order of one to three minutes, or more.

EXAMPLE 6 Another example of an excellent brightening bath is as follows (the proportions of the ingredients measured before mixing are noted under the legend Before Reacting and the proportions after mixing are noted under After Reacting):

Replenish solution In the prior art, bath compositions for chemically brightening articles of aluminum have been quite diificult to maintain for effective treatment of aluminum articles for any lenght of time. Previous methods for maintaining such baths have included determining the modifications necessary for adjusting the phosphoric, nitric acid and water by titrating. Another method included determining the modifications necessary for adjusting the nitric acid by titrating, and determining the modifications necessary for adjusting water, aluminum, and phosphoric acid by utilizing a specific gravity-viscosity chart.

In the method of maintaining bath compositions in accordance with my invention, the operating bath composition may be controlled by adding separate components to the operating bath when it needs replenishing, or by adding a replenish solution which contains all or most of the ingredients.

In my replenishing invention, unlike prior art replenish solutions, the materials for maintaining the effectiveness of the operating bath are specifically related to the composition of the starting bath.

The steps of maintaining the bath include adjusting for nitric acid and water by adding the needed quantities thereof after measuring the nitric acid in the bath by titrating or by other means, and after measuring the water. The remaining ingredients of replenishing material may be added separately at any time during operation of the bath, that is, at any degree of depletion of bath volume due to dragout. As is well-known, the volume of the bath is depleted by the amount of material which forms on the aluminum articles and is withdrawn with those articles from the bath. Also, the quantity of bath materials is reduced by evaporation, and by reaction.

The ratio of the phosphoric acid to the sulphuric acid of the replenishing mixture is the same as the ratio of those acids in the initial bath.

The specific gravity of an operating bath tends to increase during operation. The specific gravity is brought back to the desired range by adding water.

The initial bath has a dissolved aluminum content of between 1.6% and 4.0%, by weight. No further aluminum is added to the bath except that which dissolves from the aluminum articles being brightened in the bath.

The volume of the initial bath is not allowed to bedepleted by operation by more than four percent. The

replenishing solution which is added to the bath-has a specific gravity about 1.50 at 25 C. :and the ratio of the phosphoric acid to the sulphuric acid in the replenishing solution is about the same as that in the initial bath.

When my method of controlling the composition of the operating bath is followed, the concentration of dissolved aluminum is kept within a range to keep the bath from freezing, that is from becoming supersaturated with aluminum salts. My method of control also causes the bath to maintain its peak condition of brightening ability.

To prevent supersaturation of aluminum in the bath, the ratio of aluminum surface area if the article in the bath to volume of the bath is about one square foot of aluminum surface to about eleven gallons or more of operating bath. If the ratio is less than this, the bath may slowly become too rich in dissolved aluminum and be come supersaturated.

The frequency of replenishing the nitric acid and the water, before adding the other ingredients, depends mainly on the loss of nitric acid and water which takes place because of evaporation when the bath is heated but not being used to brighten aluminum articles. Additional losses are caused by dragout and by reaction.

The amount of phosphoric acid and the amount of sulphuric acid in the replenishing materials, and their ratio to one another, keeps the aluminum content in the bath in a controlled range. The ratio of phosphoric acid to sulphuric acid may drop as low as 1.8 :1 in the operating bath provided that the phosphoric acid does not go below about 35.0%, and the total phosphoric plus sulphuric acid does not drop below 30% of the phosphoric plus sulphuric acid in the initial bath.

The method of chemically brightening articles of aluminum by immersing them in a bath, and maintaining the bath for continued treatment at about constant volume, may be stated to include adding phosphoric and sulphuric acid in a weight ratio equal to that in the initial bath, also adding water to bring the specific gravity of the bath to approximately the same as in the initial bath and also by adding nitric acid to bring the percentage of nitric acid to approximately the same as in the initial bath. The initial bath may have compositions which include approximately the following ranges of active constituents.

As examples of my method of controlling and maintaining brightening baths, the following approximate proportions of ingredients, by weight, in 100 gallon initial baths are replenished by four gallon replenish solutions:

EXAMPLE 7 Initial Replenish bath solution H3P04, percent 51. 08 35. 2 H 504, percent" 27. 18. 69 HNOa, percent 3. 481 13. 193 Copper (as metal), percent. 0. 0122 0. 0053 Ammonia, percent 0.258 0. 208 H2O, percent 13. 412 31. 72 Aluminum (as metal), percent 1. 6 None Ratio of HaPO4 to 111504...- 3:1 3:1 Specific gravity (at 25 0.)-.- 1. 745 1. 51 Total H3P04 and H2SO4, percent 51.08 35. 2

EXAMPLE 8 Initial Replenish bath solution 1131 04, percent.- 571. 36 39.142 HiSOi, percent. 20. 41 3 14. 130 HNOs, percent... 3. 370 12.010 Copper (as metal), percent-.. 0. 0065 0.005 Ammonia, percent 0. 343 0. 282 H2O, percent 13. 245 32. 840 Aluminum (as metal) percent.-. 1. 39 None Ratio of wt of HZiPOA to H1804 2. 8:1 2. 7:1 Specific gravity (at 25 C.) 1. 74 1. 50

EXAMPLE 9 Initial Replenish bath solution HsPOi, percent 53. 2 41. H2804, percent 18. 30 11.67 HNOK, percent 3. 3'9 13.00 Copper (as metal), percen 0. 0120 0.0052 Ammonia, percent 0. 368 0. 460 H2O, percent 16. 31. 90 Aluminum (as metal), percent-.- 1. 595 N one Specific gravity (at 25 C.) 1. 745 1. 50

EXAMPLE 10 Initial Replenish bath solution H3PO4, percent 64. 60 46. 40 HNOs, Dercent.. 3. 50 12. 95 H1804, percent 9. 97 7.13 Ammonia, percent 0.262 0. 460 Aluminum (as metal), percent 1. 658 None H20, percent 13. 70 31.90 Copper (as metal), percent 0. 0128 0.0053 Specific gravity (at 25 C.) 1. 770 l. 50

EXAMPLE 11 Brightening baths have been maintained by adding a replenish solution when the bath becomes depleted by about 4% by volume, or less. The ranges of constituents in the initial bath and replenish solution are as follows, by weight.

The baths and the replenishing solutions of the present invention are comparatively inexpensive because of the small amount of phosphoric acid which is used.

Another advantage of the present invention is that the baths, because of the combination of ingredients and because of the specific gravity of the bath, provide that the aluminum dissolution rate, and the amount of aluminum lost by dragout, are such that the bath never becomes supersaturated with aluminum. Therefore, the customary discarding of the bath after a period of time is eliminated, thus effecting a considerable saving in cost.

Another advantage of the invention is. the saving of the expensive bath solution which is made possible by the longer transfer time which may be used, if desired. In prior art baths, the brightened article had to be moved quickly to a rinse bath to avoid etching, and there was no time to lift the article out of the brightening bath and hold it to let the dragout solution drain back. In the present invention, the article may be held for drainage of the dragout solution back into the bath without fear of etching.

What is claimed is:

1. The method of chemically brightening a surface of aluminum comprising treating the surface with an acidic mixture including, by weight, approximately 40% to 64.6% of phosphoric acid, approximately 9.97% to 25% of sulphuric acid, approximately 1.5% to 4.0% of nitric acid, approximately 1.45% to 3.1% of aluminum, approximately 13.7% to 21.5% of water, and approximately 0.0116% to 0.0126% of copper, the ratio of phosphoric to sulphuric acid being about 1.57:1 to 6.511, the total phosphoric and sulphuric acid being about 59.0% to 74.0% of the mixture, the specific gravity of the mixture at 25 C. being about 1.72 to 1.77.

2. The method of treating a surface of aluminum comprising subjecting said surface to the action of an acidic mixture having a solvent action on said surface to produce a macroscopically uniform smooth finish, said mixture including as its essential constituents, by weight, approximately 42.5% to 48% of free phosphoric acid, approximately 21% to 25% sulphuric acid, approximately 3% to 3.6% of free nitric acid, approximately 15.6% to 19% of water, approximately 1.6% to 2.7% of aluminum, and approximately 0.0116% to 0.0126% of copper, the ratio of phosphoric to sulphuric acid being about 1.57:1 to 2.07:1, the total phosphoric and sulphuric acid by weight being about 63.4% to 72.2% of the mixture, the specific gravity of the mixture at 25 C. being about 1.74 to 1.75.

3. The method of chemically brightening a surface of aluminum which comprises immersing said surface in a bath comprising, by weight, about 40.0% to 64.6% of phosphoric acid, about 9.97 to 25.0% of sulphuric acid, about 1.5 to 4.0% of nitric acid, about 1.45 to 3.1% of aluminum, about 13.7 to 21.5% of Water, and about 0.0116% to 0.0126% of copper. the ratio of phosphoric to sulphuric acid being about 1.57:1 to 65:1, the total phosphoric and sulphuric acid being about 50.05 to 89.6%, the specific gravity of the bath being about 1.72 to 1.77 at 25 C.

4. The method of chemically brightening articles of aluminum which comprises immersing said articles in a bath comprising, by weight, about 40.0 to 64.6% of phosphoric acid, about 9.97 to 25.0% of sulphuric acid, about 1.50 to 4.0% of nitric acid, about 1.45 to 3.1% of aluminum, about 13.20 to 21.5% of Water, and about 0.0116% to 0.0126% of copper, the ratio of phosphoric to sulphuric acid being about 1.57:1 to 6.5:1, the total phosphoric and sulphuric acid being about 59.0 to 74.0%, the specific gravity of the bath being about 1.72 to 1.77 at 25 C., and maintaining the bath for continued treatment of said articles at about constant volume by adding phosphoric and sulphuric acid in a Weight ratio equal to that in said bath, by adding water to keep the specific gravity of the bath approximately the same, and by adding nitric acid to keep the percentage of nitric acid in the bath approximately the same.

5. The method of chemically brightening articles of aluminum which comprises immersing said articles in a bath comprising, by weight, about 64.60% of phosphoric acid, about 9.97% of sulphuric acid, about 3.50% of nitric acid, about 13.70% of Water, about 0.0128% of 8 copper, about 0.262% of ammonia, about 1.558% of aluminum, and the bath having a specific gravity of about 1.770 at 25 C., and maintaining the bath for continued treatment of said articles at about constant volume by adding, in terms of weight of added materials, a replenish solution comprising about 46.40% of phosphoric acid, about 7.13% of sulphuric acid, about 12.95% of nitric acid, about 0.0053% of copper, about 0.460% of ammonia, and about 31.90% of water, the specific gravity of the replenish solution being about 1.50 at 25 C.

6. A bath for brightening aluminum comprising, by weight, about 40 to 64.6% of phosphoric acid, about 9.97 to 25% of sulphuric acid, about 1.5 to 4.0% of nitric acid, about 1.45 to 3.1% of aluminum, about 13.7 to 21.5% of water, and about 0.0116% to 0.0126% of copper, the ratio of phosphoric to sulphuric acid being about 1.57:1 to 6.5 :1, the total phosphoric and sulphuric acid being about 59.0% to 74.0%, the specific gravity of the bath being about 1.72 to 1.77 at 25 C.

7. A replenish solution for maintaining a bath for chemically brightening articles of aluminum, comprising, by weight, about 35.2 to 46.40% of phosphoric acid, about 7.13 to 18.69% of sulphuric acid, about 12,010 to 13.193% of nitric acid, about 0.005 to 0.0053% of copper, about 0.208 to 0.460% of ammonia, and about 31.72 to 32.840% of water, the specific gravity of the replenish solution being about 1.50 at 25 C.

8. A method of maintaining a brightening bath having, by weight, about 40 to 64.6% of phosphoric acid, about 9.97 to 25% of sulphuric acid, about 1.5 to 4.0% of nitric acid, about 1.45 to 3.1% of aluminum, about 13.7 to 21.5% of water, and about 0.01 16% to 0.0126% of copper, the ratio of phosphoric to sulphuric acid being about 1.57:1 to 6.511, the total phosphoric and sulphuric acid being about 59.0% to 74.0%, the specific gravity of the bath being about 1.72 to 1.77 at 25 C., the maintenance comprising adding, When the bath becomes depleted by less than about 4% by volume, a replenish solution comprising, by weight, about 35.2 to 46.40% of phosphoric acid, about 7.13 to 18.69% of sulphuric acid, about 12.010 to 13.193% of nitric acid, about 0.005 to 0.0053% of copper, about 0.208 to 0.460% of ammonia, and about 31.72 to 32.840% of water, the specific gravity of the replenish solution being about 1.495 to 1.515 at 25 C.

References Cited UNITED STATES PATENTS 3,119,726 l/1964 King et a1. 156-19 3,202,612 8/1965 Nelson 25279.2 3,301,787 1/1967 Cohn 25279.4

JACOB H. STEINBERG, Primary Examiner.

US. Cl. X.R. 

1. THE METHOD OF CHEMICALLY BRIGHTENING A SURFACE OF ALUMINUM COMPRISING TREATING THE SURFACE WITH AN ACIDIC MIXTURE INCLUDING, BY WEIGHT, APPROXIMATELY 40% TO 64.6% OF PHOSPHORIC ACID, APPROXIMATELY 9.97% TO 25% OF SULPHURIC ACID, APPROXIMATELY 1.5% TO 4.0% OF NITRIC ACID, APPROXIMATELY 1.45% TO 3.1% OF ALUMINUM, APPROXIMATELY 13.7% TO 21.5% OF WATER, AND APPROXIMATELY 0.0116% TO 0.0126% OF COPPER, THE RATIO OF PHOSPHORIC TO SULPHURIC ACID BEING ABOUT 1.57:1 TO 6.5:1, THE TOTAL PHOSPHORIC AND SULPHURIC ACID BEING ABOUT 59.0% TO 74.0% OF THE MIXTURE, THE SPECIFIC GRAVITY OF THE MIXTURE AT 25*C. BEING ABOUT 1.72 TO 1.77. 